Hot water systems of one form or another are installed in the vast majority of residential and business premises in developed countries. In some countries, the most common energy source for the heating of water is electricity.
Of course, as it is generally known, the generation of electricity by the burning of fossil fuels significantly contributes to pollution and global warming. For example, in 1996, the largest electricity consuming sector in the United States were residential households, which were responsible for 20% of all carbon emissions produced. Of the total carbon emissions from this electricity-consuming sector, 63% were directly attributable to the burning of fossil fuels used to generate electricity for that sector.
In developed nations, electricity is now considered a practical necessity for residential premises and with electricity consumption per household growing at approximately 1.5% per annum since 1990 the projected increase in electricity consumption for the residential sector has become a central issue in the debate regarding carbon stabilisation and meeting the goals of the Kyoto Protocol.
From 1982 to 1996 the number of households in the United States increased at a rate of 1.4% per annum and residential electricity consumption increased at a rate of 2.6% per annum for the same period. Accordingly, the number of households in the United States is projected to increase by 1.1% per annum through to the year 2010 and residential electricity consumption is expected to increase at a rate of 1.6% per annum for the same period.
It was estimated in 1995 that approximately 40 million households worldwide used electric water heating systems. The most common form of electric hot water heating system involves a storage tank in which water is heated slowly over time to a predetermined temperature. The water in the storage tank is maintained at the predetermined temperature as water is drawn from the storage tank and replenished with cold inlet water. Generally, storage tanks include a submerged electrical resistance-heating element connected to the mains electricity supply whose operation is controlled by a thermostat or temperature-monitoring device.
Electric hot water storage systems are generally considered to be energy inefficient as they operate on the principle of storing and heating water to a predetermined temperature greater than the temperature required for usage, even though the consumer may not require hot water until some future time. As thermal energy is lost from the hot water in the storage tank, further consumption of electrical energy may be required to reheat that water to the predetermined temperature. Ultimately, a consumer may not require hot water for some considerable period of time. However, during that time, some electric hot water storage systems continue to consume energy to heat the water in preparation for a consumer requiring hot water at any time.
Of course, rapid heating of water such that the water temperature reaches a predetermined level within a short period of time enables a system to avoid the inefficiencies that necessarily occur as a result of storing hot water. Rapid heating or “instant” hot water systems are currently available where both gas, such as natural gas or LPG (Liquefied Petroleum Gas) and electricity are used as the energy source. In the case of natural gas and LPG, these are fuel sources that are particularly well suited to the rapid heating of fluid as the ignition of these fuels can impart sufficient thermal energy transfer to fluid and raise the temperature of that fluid to a satisfactory level within a relatively short time under controlled conditions.
However, whilst it is possible to use natural gas fuel sources for the rapid heating of water, these sources are not always readily available. In contrast, an electricity supply is readily available to most households in developed nations.
There have been previous ineffective attempts to produce an electrical “instant” hot water system. These include the hot wire and the electromagnetic induction systems. The hot wire “instant” hot water system has been developed wherein a wire is located in a thermally and electrically non-conductive tube of a relatively small diameter. In operation, water passes through the tube in contact with or in very close proximity to the wire, which is energised to thereby transfer thermal energy to the water in the tube. Control is generally affected by monitoring the output temperature of water from the tube and comparing it with a predetermined temperature setting. Dependent upon the monitored output temperature of the water, a voltage is applied to the wire until the temperature of the water reaches the desired predetermined temperature setting. Whilst this type of system avoids the energy inefficiencies involved with the storage of hot water, it unfortunately suffers a number of other disadvantages. In particular, it is necessary to heat the wire to temperatures much greater than that of the surrounding water. This has the disadvantageous effect of causing the formation of crystals of dissolved salts normally present in varying concentrations in water such as calcium carbonate and calcium sulphate. Hot areas of the wire in direct contact with the water provide an excellent environment for the formation of these types of crystals which results in the wire becoming “caked” and thus reducing the efficiency of thermal transfer from the wire to the surrounding water. As the tube is generally relatively small in diameter, the formation of crystals can also reduce the flow of water through the tube. In addition, hot wire type systems require relatively high water pressures for effective operation and thus these systems are not effective for use in regions that have relatively low water pressure or frequent drops in water pressure that may occur during times of peak water usage.
The electromagnetic induction system functions like a transformer. In this case currents induced into a secondary winding of the transformer cause the secondary winding to heat up. The heat generated here is dissipated by circulating water through a water jacket that surrounds the secondary winding. The heated water is then passed out of the system for usage. Control is generally affected by monitoring the output temperature of water from the water jacket and comparing it with a predetermined temperature setting. Dependent upon the monitored output temperature of the water, voltage applied to the primary winding can be varied, which varies the electric currents induces in the secondary winding until the temperature of the water reaches the desired predetermined temperature setting. Whilst this type of system avoids the energy inefficiencies involved with the storage of hot water, it also suffers a number of other disadvantages. In particular, it is necessary to heat the secondary winding to temperatures greater than that of the surrounding water. This has the same effect of causing the formation of crystals of dissolved salts as discussed above. As the gap between the secondary winding and the surrounding water jacket is generally relatively narrow, the formation of crystals can also reduce the flow of water through the jacket.
In addition, the magnetic fields developed and the high currents induced in the secondary winding may result in unacceptable levels of electrical or RF noise. This electrical or RF noise can be difficult to suppress or shield, and affects other electromagnetic susceptible devices within range of the electromagnetic fields.
It is therefore desirable to provide apparatus for rapid heating of fluid, particularly water, using electrical energy and which obviates at least some of the disadvantages of other systems.
It is also desirable to provide an improved method for rapidly heating water using electrical energy which minimises power consumption.
It is also desirable to provide an improved system for heating water using electrical energy which provides relatively rapid water heating suitable for domestic and/or commercial purposes.
It is also desirable to provide an improved apparatus and method for electric fluid heating which facilitates control of the output temperature whilst minimising formation of crystals of dissolved salts.
It is also desirable to provide an improved fluid heating system which uses mains power generally available in domestic and commercial buildings.
It is also desirable to provide an improved heating apparatus which can be manufactured in various capacities of fluid throughput.
It is also desirable to provide fluid heating apparatus which can be designed to operate with a variety of fluids or with water of varying hardness.
It is also desirable to provide fluid heating apparatus which can be installed in close proximity to the hot water outlet, thereby reducing the time delay of the arrival of hot water and thereby obviating unnecessary wastage of water.
It will be understood that any discussion of devices, articles or the like which has been included in this specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters either form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.